This invention relates generally to an improved electron gun for television receiver cathode ray tubes, and is specifically addressed to an improved focus lens electrode structure that provides beam convergence substantially independently of any beam-focus-related adjustments in the main focusing fields and without inducing significant beam distortion. This invention has applicability to guns of many types and constructions, but is believed to be most advantageously applicable to three-beam unitized electron guns for color television cathode ray tubes.
Unitized electron guns for color cathode ray tubes generate three electron beams developed by cathodic thermionic emission. The resulting beams are formed and shaped by a tandem succession of electrodes spaced along the central axis of the gun. The electrodes cause the beams to be focused on multiple phosphor groups located on the faceplate of the containing color cathode ray tube. A prime objective in the design of such guns is to provide small electron beam spot size with enhanced picture resolution even at high beam currents.
Primarily for cost reasons, the current trend in color television receiver design is toward color tubes with in-line guns and stripe screens. Such tubes permit substantial simplification of convergence-related tube hardware and receiver circuitry. Gun unitization; i.e., the use of common structures for different gun parts, permits further economies.
The three electron beams of an in-line gun lie side-by-side in the same plane. The inner beam proceeds on a straight line path down the center axis of the gun and through the gun toward its landing point on the phosphor-bearing faceplate of the cathode ray tube. The two outer beams, however, which are desirably (for space economization) parallel to the center beam, must be diverted inwardly, that is, from the straight line paths at some point within the gun so that they converge at a landing point common with the inner beam. This converging of the two outer beams must be accomplished without distorting the beams. If the beams are distorted (assuming them to be initially circular in cross-section), they will no longer be circular in cross-section at the point of landing, but elliptical.
Prior art structures for converging electron beams have relied upon a variety of techniques such as the use of magnetic influences within and/or without the tube envelope, and the use of electrostatically charged plates. Also, the prior art shows many examples of inducing beam diversion or convergence by inducing an asymmetry in an electrostatic field formed at the interface of two spaced electrodes. Such an asymmetrical electrostatic field appears to have invariably induced a distortion in the shape of the beam with resulting reduced picture resolution. Prior art techniques for inducing electrostatic field asymmetry having included offsetting the opposing faces of two electrodes, slanting one or more of the opposing faces, or shaping the opposing faces so that the space lying between is in the form of a wedge. The result has been an inevitable distortion of the beam -- a result which is more or less tolerable depending upon the standard of picture quality accepted.
A simple means to effect beam convergence in multiple beam guns is by simply physically tilting each gun so that all beams fall upon a common point of convergence. This has been a common practice in delta-configured guns used in large neck cathode ray tubes because the space is available for such tilting. Also, the triangular configuration makes optimum use of the cylindrical space available in the tube neck.
This simple means of obtaining convergence by tilting the guns is not feasible for in-line guns however by virtue of the fact that the guns lie side-by-side, and thus do not make as effective use of the available neck space. Also, the neck space problem has been exacerbated through the years in that as beam deflection angles have increased, the available space within the neck of cathode ray tubes has been diminished. As an example of this trend, the in-line unitized gun that is the subject of this disclosure has an outside diameter of less than an inch. As a result of this space restriction, it is inconvenient to tilt the guns to achieve convergence, so convergence must be accomplished by other means.
As will be described hereinafter, this invention concerns an electron gun having improved electrostatic beam diversion structure -- especially a three-beam gun for color cathode ray tubes having improved structure for accomplishing beam convergence.
As mentioned, the prior art shows many examples of electrostatic beam diversion or convergence structures. U.S. Pat. No. 3,889,146, for example, teaches a convergence system for a delta-configured three-beam color cathode ray tube electron gun, in which beam-passing apertures of a focusing grid are made larger and are radially offset from the apertures of a preceding grid to effect convergence of the three beams.
A structure for electrostatic beam convergence in a unitized, in-line electron gun having a bipotential lens is described in U.S. Pat. No. 3,873,879. Beam focusing is accomplished by the provision of a large potential difference at the gap between the two focus grids. Convergence of the two outer beams toward a common landing point with an inner beam is accomplished by the outward offsetting of two outer beam apertures of the second focus lens grid relative to the beam path. This outward offsetting of the apertures produces asymmetrical focusing fields which cause the outer electron beams to be converged. To compensate for elliptical distortion of the outer beams caused by the asymmetrical converging-focusing field, a concavity is provided in the face of the down-beam grid.
German Patent No. 2,406,443 shows a beam convergence scheme for an electron assembly having three discrete guns. Each gun has a bipotential electron lens. The off-axis lenses have opposed end faces on either or both the focusing electrode and the anode which are inclined relative to the axis of the gun assembly to cause the beams to converge. In passing through such a structure, beam shape is said to be altered from round to elliptical, causing astigmatism (marginal blur) of the beams so diverted. The alleged invention in that patent lies in providing elliptically shaped electrode apertures to neutralize the ellipticity of the beams.
In U.S. Pat. No. 2,957,106, a system of beam convergence is recited wherein beams are caused to converge at the low-voltage (cathode) end of a bipotential electron gun. In one embodiment, the beam apertures are offset radially. In another, the opposing faces of two adjacent electrodes in the low-voltage area are caused to have a similar angle (as shown by FIG. 5 of the cited disclosure), and the convergence attained is a function of electrode face angle and the relative potential of the two electrodes.
The disadvantages believed to be inherent in the U.S. Pat. No. 2,957,106 system are three-fold: (1) convergence of beams by passing them through an asymmetrical field early in their travel through the gun results in substantial and largely irremediable astigmatism; (2) such early convergence makes manufacture of multi-beam guns extremely difficult in that the beam channels of electrodes that follow the convergence electrodes must also tilt inwardly, and the faces of all succeeding electrodes must be slanted so as to be perpendicular to the axis of each beam to avoid further distortion of the beam. This inward tilting is progressive, and results in a physical "squeezing" of the components in the progression of the components from the point of convergence to the end of the gun. Such squeezing results in a progressive decrease in barrel lens diameter and hence in increased spherical aberration.
Also, the ever-closer adjacency of the gun parts encourages beam-to-beam interaction, especially in an aperture lens configuration. Thirdly, there is an interaction between focus and convergence. The convergence means is focus-voltage-sensitive and G2 voltage-sensitive so that the resulting angle of convergence is susceptible to gun-to-gun focus voltage differences due to constructional variations and G2 voltage variations, and also due to gun-to-gun variations in the cathode-to-G2 spacing. The resulting range of misconvergence values due to focus voltage and G2 voltage variation in manufacture of the cited convergence method could be quite large depending upon the actual design values of focus voltage and G2 voltage. Further, the approach taught in the cited disclosure is not believed to be amenable to unitization.
Beam convergence can also be accomplished by means of post-focus electrostatic or magnetic convergence plates placed parallel to the beam travel. Or, magnetic pole pieces adjacent to the beams at the emission end of the gun can exert a converging influence on passing beams. Such approaches, however, add cost, lengthen the gun (and thus the cathode ray tube), and are apt to distort the beams.
______________________________________ Other Prior Art Beam Diversion by Beam Convergence by Asymmetric Field Asymmetric Field ______________________________________ U.S. 2,496,127 U.S. 3,889,146 U.S. 2,638,559 U.S. 3,890,528 U.S. 2,792,515 German 2,500,818 U.S. 2,884,551 German 2,358,896 U.S. 2,911,563 Convergence by Plate Structure or Pole Pieces Separate from the Focus Lens U.S. 2,849,647 U.S. 3,866,080 U.S. 3,571,645 U.S. 3,579,008 U.S. 3,614,500 U.S. 3,614,501 U.S. 3,619,687 U.S. 3,678,318 ______________________________________